A 16-year-old girl developed pyoderma gangrenosum without underlying disease at first. She was treated with local cutaneous dressing and systemic oral prednisolone and minocycline for 10 months, imtermittently. After that, acute leukemia, which was undeterminated type, was developed 4 years after first onset. Since pyoderma gangrenosum may be a preindication of underlying disease, continual follow up check for systemic disease is required.